Pollution in the NIR
To quantify the possible polluting effect of artificial illumination we have measured the NIR background at different altitudes (ALT) and azimuth (AZ).
All measures were obtained during dark time to avoid the effect of the moon. The values relative to the zenith background are plotted in Fig. 1. Blue, green and red lines represent the J, H and Ks filters respectively. The solid lines are measures at ALT=20, the dashed lines at ALT=45. The brightest artificial spots on the La Silla horizon are produced by the cities of La Serena, about 85 Km to the south-west (AZ=30) and Vallenar about 75 Km to the north (AZ=150). The approximate positions of the two towns are marked in Fig. 1.
There is no correlation between these positions and the variation of the NIR sky background. The pick at AZ about 330 is related to the Galactic Center that was rising at that position during the observations. Apart from this the most obvious trend is a maximum toward the north (AZ=180) and a minimum toward the S-W, direction of the coast.